Aqueous suspensions of powdered inorganic or organic substances such as clays, porcelain slurries, silicate dust, chalk, soot, stone dust, pigments, talcum, powdered plastics and hydraulic binders are in practice often mixed with additives to improve their workability , i.e. their kneadability, spreadability, sprayability, pumpability or their flow properties. These additives, which usually contain ionic groups, are able to break up the agglomerates formed when water is added to these substances and to electrostatically and/or sterically stabilize the resulting solid particles. The internal frictional forces are reduced considerably as a result, and the working properties improved accordingly.
This effect plays a particularly important role in the production of building material mixtures which contain hydraulic binders such as cement, lime, gypsum or anhydrite. In order to obtain these building material mixtures in a ready-to-use and workable form it is as a rule necessary to use a lot more water for mixing than is actually required for the subsequent hydration and hardening process. This excess water evaporates later, resulting in a sizeable void fraction in the finished construction. This in turn leads to significantly poorer mechanical strength and resistance.
Through use of the additives described above it is possible to lower the quantity of excess water and/or to improve the working properties for a given water/binder value. As water-reducing agents or superplasticizers of this kind melamine formaldehyde polycondensation products containing sulfonic groups, as described for example in the DE-PS 16 71 017, are particularly well known, as also formaldehyde polycondensation products of naphthalene or alkylnaphthalene sulfonic acid as according to the EP-PS 214 412. The disadvantage of these additives is the fact that the excellent water-reducing or plasticizing effect only lasts for a short period and that through the use of formaldehyde as condensation component the products always contain a certain proportion of free formaldehyde, which ranks as a toxicological risk. The EP-A- 558 336 describes the use of non-mineral organic components based on hydroxymethylated amino carboxylic or aminosulfonic acids to accelerate setting. When used alone these synthetic products accelerate setting only slightly, having a significant effect only when used in combination with inorganic accelerators. What is more of a disadvantage, however, is the fact that these products, because of the way they are made, also contain formaldehyde.
Formaldehyde-free dispersing agents based on acrylic acid (cf. DE-OS 35 29 095), maleic monoesters and styrene (cf. EP-A 306 449) as well as alkyl polyethylene glycol monoallyl ethers and maleic anhydride (cf. EP-A 373 621) are able to prolong the workability of hydraulically setting building material mixtures for a suitable length of time, but often have the disadvantage of retarding the setting process.
Last but not least, polycondensation products of melamine and glyoxylic acid are known from the DE-OS 41 17 181 which can prolong the time of workability of cement-based building material mixtures without noticeably retarding the setting process.
The disadvantage of these polycondensation products is that their effect is limited to Portland cement containing flue ash and that they are required in relatively large quantities, which means that the cement takes longer to set than do cement mixtures which are free of additive It is true that this disadvantage can be overcome by adding to the building material mixtures in question additional additives such as calcium chloride to accelerate setting, but the use of such additives is again associated with disadvantages. Calcium chloride, for example, even in extremely low concentrations, has a highly corrosive effect on reinforcing steels used in time-tested building material mixtures. Alkali and alkaline earth hydroxides, carbonates, aluminates, silicates or fluorosilicates, which in the form of solutions are likewise known as setting accelerators, have very high pH values and if they leach out can cause ecological problems. The handling of these accelerators in powder form can lead to industrial hygiene problems. The hazard potential for man and the environment of alkaline earth metal nitrates or nitrites, lastly, which again are familiar accelerators, is sufficiently well known.